1. Field of the Invention
The present invention relates to an anode having an anode active material layer on an anode current collector and a secondary battery including the anode.
2. Description of the Related Art
In recent years, portable electronic devices such as video cameras, mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as an electric power source for the portable electronic devices, a battery, in particular a light-weight secondary batter capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a secondary battery provides a higher energy density than a lead battery and a nickel cadmium battery.
The lithium ion secondary battery includes a cathode, an anode, and an electrolytic solution. The anode has an anode active material layer on an anode current collector. The anode active material layer contains an anode active material contributing to charge and discharge reaction. The anode active material layer may contain other material such as an anode binder according to needs. It is known that fine pores (void) of varying size exist in the anode active material layer as described in, for example, Japanese Unexamined Patent Application Publication Nos. 2005-293899 and 2004-071305.
As the anode active material, a carbon material has been widely used. However, in recent years, as the high performance and the multi functions of the portable electronic devices are developed, further improvement of the battery capacity is demanded. Thus, it has been considered to use silicon or the like instead of the carbon material. Since the theoretical capacity of silicon (4199 mAh/g) is significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby highly improved.
In the case where silicon powder is used as an anode active material, coating method, sintering method or the like is used as a method of forming an anode active material layer as described in, for example, Japanese Unexamined Patent Application Publication Nos. 11-339777 and 11-339778. In the coating method, after slurry containing the anode active material, an anode binder and the like is prepared, the surface of an anode current collector is coated with the slurry and then the resultant is dried. In the sintering method, after the surface of the anode current collector is coated with the foregoing slurry and the resultant is dried, heat treatment (firing) is provided. In the case where the coating method or the sintering method is used, pressing is performed after slurry coating according to needs.
However, in the case where silicon is used as an anode active material, the anode active material layer is intensely expanded and shrunk in charge and discharge. Thus, the anode active material layer may be dropped from the anode current collector. Further, in charge and discharge, adjacent anode active materials collide each other to generate strain (internal stress) in the anode active material layer. Thus, the apparent thickness of the anode active material layer may be increased. Thereby, while a high capacity of the battery capacity is obtained, there is a possibility that the cycle characteristics as important characteristics of the secondary battery are lowered and the thickness thereof is unintentionally increased.